In underwater photography cameras originally designed for use on land are housed in various kinds of underwater housings in order to protect the cameras against water pressure at depths to as much as 90 meters or more. There are two main categories of cameras currently in use by underwater photographers:
1) Simple digital cameras normally referred to as ‘point and shoot’ camera, in which most users frame (compose) the pictures by looking at the rear LCD display; and
2) Single Lens Reflex cameras, in which users frame the pictures by viewing through the viewfinder of the camera.
When using the point and shoot cameras for underwater photography, users can always frame a picture from the LCD display of the camera, through a transparent window integrated into the underwater housing. The users can normally see a relatively clear display of the picture without any modification of the housing. However, the function of this kind of cameras is rather limited as the cameras are not designed with lens interchangeability to allow photographers to use different lenses.
A more serious underwater photographer utilizes different kinds of lenses for achieving different photographic effects. For example, some lenses known as macro lenses provide very close focusing capability for photographing extremely small objects; and some lenses provide extreme wide angle capability for wide angle scenes and big objects. Single Lens Reflex cameras are designed for this kind of lens interchangeability.
As mentioned, the Single Lens Reflex cameras (SLR cameras) are normally installed inside a specially built housing for underwater photography. The photographer looks through the viewfinder of the camera in order to frame his picture.
With the SLR camera installed in an underwater housing, the photographer positions his eye behind a window on the housing in order to view through the viewfinder of the SLR camera. However, as the housing is some distance away from the back of the camera; and the photographer has to wear a diving mask, his eye could be some 60 mm to 70 mm away from the ocular lens of the SLR camera viewfinder.
The SLR camera has a viewfinder system design such that the photographer sees a virtual image of the ground glass viewscreen of the SLR camera through the ocular lens. The image of the picture from the objective lens at the front of the SLR camera is firstly projected onto this ground glass viewscreen. Hence the photographer, when framing for a picture, sees an image equal to that projected onto the film (or sensor in case of a digital SLR) at the time when the camera is taking a picture.
The normal viewfinder system of a SLR camera allows a viewing distance of about 20 mm to 25 mm from the ocular lens. When the photographer has his eye positioned further away, as mentioned above in the case of taking underwater photographs using the SLR camera in an underwater housing, the photographer is unlikely to see the whole image through the ocular lens.
Hence, most underwater housing designed for SLR cameras have a wide angle viewer, which is similar to a door viewer, in order to bring a full view of the image to the user. In the case of looking through a door viewer, the image is very much compressed so as to allow a wider view of the outside image to pass through the viewer. Similarly, in the case of underwater photography using a SLR camera installed in an underwater housing with a wide angle viewer, the image is compressed and looks very small to the photographer. This presents a high degree of difficulty for the photographer who cannot clearly see the image from the SLR camera.
A device known as Enhanced Viewfinder has been designed to overcome this difficulty. An enhanced viewfinder utilizes a combination of terrestrial telescope and prism(s) to bring the view of the image from the SLR camera forward to the eyepiece so that the photographer has a view similar to that from the SLR ocular lens. But the design of conventional enhanced viewfinder does not allow for adjustment of eyesight while the photographer is using the camera underwater.
It is a common phenomenon that people have eyesight differences especially with advancing age. Some people are long-sighted while some are short-sighted. The SLR camera viewfinder system provides adjustment for eyesight compensation by moving the ocular lens forward or backward, which in effect adjusts the distance of the virtual image of the ground glass viewscreen from the ocular lens to compensate for different eyesight.
However once the SLR camera is installed inside an underwater housing, it cannot be adjusted from the outside of the underwater housing anymore. Certain enhanced viewfinders do provide a means to adjust for eyesight differences, but the designs are such that these adjustments have to be done prior to the dive by disassembling certain parts, and re-assembling them after the adjustment. This presents two problems:    1. Users cannot see the result of adjustments instantly. It always takes repeated disassembling, adjustment and then re-assembling until the result is acceptable.    2. The adjustments cannot be done while the enhanced viewfinder is in use underwater.